Positive reversal process



Oct. 16, 1945. H. w. HOUSTON 2,386,858

POS ITIVE REVERSAL PROCES 5 Filed April 21, 1943 l/vvz/v-rol? Q2 HEPBEPTWHOUSTON Y HA PIP/5, K/ch; P05 715/? @zHA RAJ/S FOR THE F/YM ATTORNEYS.

Patented Oct. 16, 1945 POSITIVE REVERSAL PROCESS Herbert W. Houston, Sherman Oaks, Calii., assignor to The Houston Corporation, Los Angeles, Calif., a corporation of California Application April 21, 1943, Serial No. 483,917

9 Claims.

My invention relates to photographic methods, with special reference to film development and is directed specifically to an improved process for producing positive images by reversal.

It is an object of my invention to provide a reversal process effective to produce a positive image of quality over a wide range of departure from normal camera exposure, it being a special object of the invention to extend the range in the direction of over-exposure, thereby to utilize film heretofore regarded as over-exposed beyond retrieve.

Another object of the invention is to provide a reversal process that will produce a higher average quality than heretofore attained when applied to the development of numerous positive reversal films from various sources exposed under diverse circumstances with widely varying degrees -of skill. It is proposed to provide a reversal process that may be applied indiscriminately to variously exposed film to provide inherently and automatically compensation for all deviations from normal exposure.

It is a further object of the invention to provide a reversal process which is not critical in operation and which does not necessitate employment of photoelectric control to meet the varying requirements of variably exposed film.

A still further object of my invention is to provide a procedure for developing a positive image involving the use of successive light exposures in such manner as inherently to provide compensation for various degrees of camera exposure of the film.

In some practices of my invention it is my further object to favor the availability of lightsensitive halide grains in the film to build up the density of the final positive image, especially in the processing of grossly over-exposed film. More specifically, I propose to utilize in buildin up the final positive image some of the lightactivated halide grains comprising the original latent negative image.

In prior art practices heretofore prevalent, it has been considered essential to develop the exposed film initially to a relatively high gamma or substantially to completion This accepted practice characterized by initial development to a high gamma is not satisfactory for over-exposed film. Any degree of over-exposure in the camera causes disproportionate loss in quality in the fina positive image, and substantial over-exposure usually results in a completely worthless finished product.

It has heretofore been assumed that after the a later stage in the reversal process to negate the.

final positive image. One object of my invention is to provide a process that permits incomplete initial development of the potential negative image without involving the usual penalty of subsequently producing a developed residual negative to the material detriment of the final product.

Since initial full development followed by bleaching reduces the subsequent image-forming capacity of the film, a film that is over-exposed in the camera and then initially developed to completion in the conventional manner is inherently incapable of producing an adequate positive image and no photoelectric control can compensate for the deficiency. One of the objects of my invention is to achieve an acceptable, if not an excellent, positive image in the case of over-exposed film by making it possible to utilize for the final positive image a substantial portion of the halide grains that are initially activated in the camera as partof the initial latent negative image. I have good reason to believe that in one practice of my invention emphasizing the treatment of over-exposed film, at least some halide grains that are initially activated in the camera and survive temporarily as the residual latent negative image are actually deactivated and restored to sensitivity in the course of my process and subsequently respond to light to form density in the final positive image.

A further object of my invention, then, in the practice of my invention that includes processing of over-exposed film is to provide a bleaching agent and a breaching step that will have the effect of erasing a residual latent negative image, the term erasing being used in the sense that the bleach deactivates the grains comprising such residual image and restores the grains to lightresponsiveness for subsequent participation in a positive image. I

The above and other objects of my invention will be apparent in the following detailed descrip tion, taken with the accompanying drawing.

The drawing, which is to be considered as illustrative only, shows partly in section and partly in side elevation an apparatus that may be em- 1ployed in the preferred practice of my invenion.

In my preferred practice, the exposed direct positive film is first developed to substantially less than completion and is then passed into a stop solution to arrest development at .65-.75 gamma. The negative image produced by the initial development is then bleached out by passing the film into a special bleaching solution. In my preferred practice I employ a dichromate bleach, the proportions being 1% dichromate of potassium and one-half of 1% sulphuric acid in water solution. Dichromate of ammonium or dichromate of sodium may be substituted for the dichromate of potassium. Such a bleaching agent will effectively remove the silver grains of the developed negative image and, apparently, will also deactivate the halide grains comprising the residual latent negative image and restore such halide grains to at least some degree of their original responsiveness to light. Thus, if the original latent negative image is developed to substantially less than completion to leave a residual latent negative image and the film is treated with such a bleaching medium, halide grains comprising the residual latent negative image become available for subsequently building up density in the final positive image.

The degree to which initial development is curtailed in the creation of the initially developed negative image will vary in various practices of the invention. Since the purpose of such curtailment is to conserve some of the silver halide grains that are initially light-activated in the camera, the degree of curtailment may be determined in advance more or less precisely in accord with a known need of utilizing in the final positive image a known portion of the initially light-activated halide grains. If the degree of over-exposure in the camera is known, such a need may be closely estimated. A feature of the invention, however, is that it is not necessary to ascertain in advance what proportion of the camera-activated grains will be required in a particular instance to avoid deficiency in the final positive image, since curtailment of development of the initial negative image suflicient to take care of a maximum requirement in the positive image.

will meet all lesser requirements. In other words, a curtailment of the initial negative image development to meet the requirement of a film that is over-exposed in the camera to a given degree will meet the requirements for all films that are over-exposed in the camera to lesser degrees. Under highly favorable conditions the range of deviation from normal camera exposure maybe relatively small, and the need for curtailment of the initial negative image development correspondingly reduced. n the other hand, if the film to be processed covers a wide range of departure from normal exposure, especially in the positive image. One factor arises from the fact that the initial camera exposure of the film to produce the initial latent negative image activates a relatively large proportion of the silver halide grains in the high-light areas of the scene and affects a relatively slight proportion of halide grains in the shadow areas of thescene. Consequently, in the high-light areas there are relatively few halide grains left to respond to light and the few do not include any substantial proportion of highly sensitive halide grains. On the other hand, in the shadow portions of the scene there are relatively great concentrations of halide grains remaining for response to light, and these large concentrations include in substantial portion halide grains that are highly sensitive to light. In other words, there is not only a quantitative distinction between the high-light areas and the shadow areas, but also a qualitative distinction, the film having higher speed in the shadow areas and a corresponding slower speed in'the areas which were formerly negative image areas of low density, the speed being in exact relation to the density of the negative image. Both of the distinctions quantitative and qualitative, favor reversal, and even if all of the lightactivated grains comprising the latent negative image were removed by development and bleaching, subsequent exposure of the film to light would produce a positive image.

Another factor contributing to favorable differenti'ation in the speed of various areas of the film is that the products of development arebromides or iodides, which products are agents restraining the responsiveness of the film to light. It is known that some of these products of developing the initial negative image survive in the areas in which they are produced to favor the creation of the re'versed or positive image by reducing the 'sensitivityof'the remaining halide grains in those areas. the reduction in light sensitivity in these areas being in exact proportion to the density of the original negative image.

It is believed that a further favorable factor may be attributed to the bleaching operation and the particular bleaching agent employed in my preferred practice. It is thought that silverdirection of over-exposure, rather severe curtailment of initial negative image development will be required. For example, if the process is to be employed for commercial finishing of amateur film from diverse sources or is to be employed to finish film taken under war conditions by the armed services, substantial curtailment will be found desirable. I have found that curtailment of the development at .65-.75 gamma is satisfactory for film varying widely between gross under-exposure and gross over-exp sure.

After bleaching, the film is passed through a suitable clearing solution and then, in the preferred practice of my invention, is immediately passed under a printing lamp for the activation of halide grains to form an initial latent positive image.

There are a number of factors that may aceffect in the areas of the film which were formerly negative.

After exposure to the first printing lamp, the film is immersed in a developing solution for a period of time that is brief but is sufficient to bring out the latent positive image at least to a substantial degree. Under favorable conditions, with special emphasis on a favorable degree of exposure of the film in the camera, the latent positive image will be relatively strong and in the brief development period a positive image of adequate density will arise. On the other hand, under unfavorable conditions, with emphasis on over-exposure in the camera, the initial latent positive image will be relatively weak and only a relatively thin positive image will appear in the first brief developing period following exposure to the first printing lamp.

At the end of the brief development period following exposure to the printing lamp, the film is at least once again exposed to light and given further development. All of the additional lamps after the printing lamp may aptly be termed flashing lamps. In my preferred process, for example, exposure of the film t0 the first printing lamp may be followed by three successive exposures to flashing lamps with intervening development periods, there being four development periods. The total development time represented by the four development periods will in the case of widely diverse films be the maximum required for grossly over-exposed film to permit a relatively. weak initial positive image to be built up progressively in such film to adequate density. This total development time, which is determined by grossly over-exposed film, may be divided into four equal periods, for example, a developing period amounting to 25% of the total development time intervening between exposure to the first printing lamp and exposure to the subsequent flashing lamp, a second 25% development period intervening between the second light exposure and the third light exposure, a third 25% development period intervening between the third light exposure and'the fourth light exposure, and a final 25% development period following the last light exposure. After the last development period, the film passesthrough an acid stop and is fixed, washed and dried in the usual manner.

In practice the adjustment of the successive lamps is not critical, it being necessary merely to avoid too drastic exposure early in the series of exposures. The first or printing lamp may, if desired, be so weak that in all instances subsequent light exposure will be required to build up a positive image of adequate density. I prefer, however, to so adjust the first printing lamp that it will activate suflicient halide grains in underexposed film to cause a positive image of adequate density to appear in the following development period prior to exposure to the succeeding flashthe initially developed positive image, being of ing lamp. Under favorable conditions, then, the initially developed positive image, being of adequate density, serves as an efiective mask under all of the succeeding flashing lamps and the succeeding fiashing lamps serve no function whatsoever. If the initial positive image is of intermediate density, it will serve as a less effective mask and will permit at least the first of the succeeding flashing lamps to activate further halide grains in the film to bring the positive im age up to adequate density in the second 25% development period. If the initial positive image is quite weak, each of the succeeding flashing lamps may function to activate additional halide grains in the film, the positive image attaining adequate density only in the final 25% development period. It is apparent that the process automatically compensates for various degrees of camera exposure without the employment of photoelectric control.

In considering this preferred practice of my invention, it is apparent that some of the steps of the process are important for camera underexposed film, but unimportant for camera overexposed film, and conversely, other steps in the process are important for camera over-exposed film. Thus, film that is grossly under-exposed in the camera tends to yield a strong initial positive image in the first 25% development period,

' making the subsequent flashing lamp unnecessary. Since under-exposure of film in the camera does not seriously deplete the available lightsensitive halide grains in the film, it is not essential that the development of the preliminary negative image be stopped substantially short of completion. On the other hand, film that is grossly over-exposed in the camera tends to produce a relatively thin initial positive image so that subsequent exposure to the succesisve flashbinations that may either be brought together as described above to make a universal process, or in other practices of my invention may be used as separate processes simplified and specialized for either under-exposed film or over-exposed film.

The drawing indicates, by way of example, the construction of apparatus that may be employed for carrying out the described process in a continuous manner. The exposed direct positive film I0 is led from a reel ll over a guide roller l2 into an initial developing tank I3, the film in the tank traversing between an upper gang of rollers l5 and a lower gang of rollers [6 for a time period representing development to .65.75 gamma. The film with the developed negative image thereon crosses guide rollers l'l into a tank [8 of stop solution to arrest development. From the tank IS the fi1m crosses guide rollers I9 to pass through a tank 20 of bleach solution and then crosses guide rollers 22 to pass through a tank "containing a clearing solution.

The bleached and cleared film passes over guide rollers 25 under a printing lamp 26 and makes an initial number of traverses in a de-v veloping tank 21, which traverses represent the first 25% of development time to bring out the initial positive image. At this juncture the partially developed film is guided by rollers 28 under a flashing lamp 30 with the initial positive image serving as a mask of some degree of effectiveness, depending upon the density of the image. The film then makes sufficient additional traverses in the developing tank 21 to represent the second 25% development period and is then guided by rollers 3| past a flashing lamp 32. Again the film makes a number of traverses through the developing tank 21 to account for the third 25% development period, whereupon the film is led by guide rollers 33 past a final flashing lamp 34. After exposure to the flashing lamp 34, the film makes a final number of traverses through the developing tank 21 for the final 25% development period. The completely developed film is now passed through a tank 40 containing a stop solution, a tank 4| containing a fixing solution, and a wash tank 42. From the wash tank 42 the film is guided by rollers 43 into a suitable drying cabinet generally designated 44, the completely dried film emerging from the cabinet to be wound on a reel 45.

Since it is difiicult to ascertain precisely why my process is successful and so advantageous over prior art procedures I am not to be bound by any explanatory comment herein.

The procedures and apparatus described in detail for the purpose of disclosure and to teach the principles involved will suggest to those skilled in the art various changes and substitutions under my basic concept, and I reserve the right to all departures from my disclosure that properly lie within the scope of my appended claims.

I claim as my invention:

1. A reversal process for exposed film, includ-- ing the steps of: developing the exposed film to substantially less than completion, thereby converting a major portion of initially light-activated grains of the film to form a negative image and leaving unconverted a substantial remaining portion of initially light-activated halide grains comprising a residual latent negative image; bleaching out the converted grains comprising the said developed negative image and deactivating the said unconverted halide grains so that said latent negative image will not be converted to any material extent to a visual negative image upon subsequent development of the film; exposing the film to light to activate halide grains including at least some of said deactivated. halide grains; developing the film to convert the light-activated grains therein to form a positive image of normal density; and

fixing the film.

2. A reversal process for exposed film, including the steps of: developing the exposed film to substantially less than completion, thereby converting a major portion of the initially light-activated grains of the film to form a negative image and leaving unconverted a substantial remaining portion of the initially light-activated halide grains comprising a residual latent negative image; bleaching out the converted grains comprising said developed negative image and deactivating said unconverted halide grains so that said latent negative image will not be con verted. to any material extent to a visual negative image upon subsequent development of the film; exposing the film to light to activate halide grains to form a latent positive image of inadequate density; developing the film to bring out an initial positive image of inadequate density; and subjecting the film to at least one additional cycle of light exposure and subsequent development with the developed positive image serving as a mask in each cycle, until the developed positive image attains adequate density; and fixing the film.

3. A reversal process for exposed film as set forth in claim 1 in which a dichromate bleaching agent is employed to bleach'out the negative image and deactivate the halide grains of the residual latent negative image.

4. A reversal process for exposed film as set forth in claim 2 in which a dichromate bleaching agent is employed to bleach out the negative image and deactivate the halide grains of the residual latent negative image.

5. In a reversal process for exposed film, the steps of: developing the exposed film to substantially less than completion, to produce a visible negative image and a latent invisible negative image; bleaching the film with a dichromate bleaching agent; exposing the film to light to produce a latent positive image; redeveloping the film to render visible said positive image and to provide it with substantially normal density not requiring reduction; and fixing the film.

6. In a reversal process for exposed film, the steps of: developing the exposed film to substantially less than completion, to produce a visible negative image and a latent invisible negative image; bleaching the film with a bleaching agent adapted to deactivate at least a portion of said latent invisible negative image; exposing the film to light to produce a latent positive image; redeveloping the film to render visible said positive image without rendering visible said deactivated portion of said latent negative image as such, so that the average density of said positive image is not materially increased by the addition thereto of a redeveloped visible negative image; and fixing the film.

7. In a reversal process for exposed film, the steps of: developing the exposed film to substantially less than completion, to produce a visible negative image and a latent invisible negative image; bleaching the film with a bleaching agent adapted to reduce the response of said latent invisible negative image to subsequent redevelopment; exposing the film to light to produce a latent positive image; redeveloping the film to render visible said positive image without rendering visible said latent negative image as such, so that the average density of said positive image is not materially increased by the addition thereto of a redeveloped visible negative image; and fixing the film.

8. In a reversal process for oxer-exposed film, the steps of under-developing an exposed film, to produce a visible negative image of substantially normal density and a latent invisible negative image; treating the film with a bleaching agent so as to bleach out said visible negative image and to erase at least a substantial portion of said latent negative image; exposing the film to light to produce a latent positive image formed in part of some of the halide grains which previously formed said erased portion of said latent negative image; redeveloping the film to render, visible said positive image and to 'provide it with normal density not requiring reduction; and fixing the film.

9. In a reversal process for over-exposed film, the steps of: under-developing an exposed film, to produce a visible negative image of substantially normal density and a latent invisible negative image; treating the film with a bleaching agent so as to bleach out said visible negative image and so as to resensitize to light at least a portion of the halide grains which previously formed said latent negative image; exposing the film to light to produce a latent positive image made up at least in part of said resensitized halide grains; redeveloping the film to render visible said positive image and to provide it with normal density not requiring-reduction; and fixing the film.

HERBERT W. HOUSTON.

cERTIFIcAjE 0F CORRECTION. Patent Non, 2,586,858., October 16, 191

- HERBERT w. .HOUSTONQ It is hereby certified that error a apears in the printed specification of the above numbered patent requiring correction as follows: Page 5, firstcolumn, line 56, strike out the words "the initially developed positive image, being of"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5th day of February, A. D. 19Me Leslie Frazer (Seal) First Assistant Commissioner of Patents 

